Ring oscillator circuit for VCO with frequency-independent duty cycle

Abstract: In a ring oscillator type VCO in which plural stages of inverter circuits (10) are cascade-connected to each other so as to constitute a positive feedback loop, both of delay amounts for a rising edge and a falling edge of an output signal from the inverter circuit are controlled to have the same delay amount by way of a control signal (Vc). These delay amounts of the rising edge and the falling edge are controllable in such a manner that the duty ratio of an oscillator output signal is not varied. Each stage (10) of the inverter circuit is arranged by three-staged inverters made of load transistors (Q L ) and driver transistors (Q D ), and the control voltage (Vc) is applied to the load transistors of the two adjoining inverters among the three-staged inverters.

Description: Background of the Invention 1. Field of the Invention The present invention generally relates to a ring oscillator type VCO (voltage-controlled oscillator). More specifically, the present invention is directed to a ring oscillator type VCO capable of controlling a phase of an oscillated output signal based upon a control voltage without varying a duty ratio thereof. 2. Description of the Related Art As a voltage-controlled oscillator (abbreviated as "VCO"), there is known a ring oscillator type VCO using inverter blocks as represented in Fig. 1A and Fig. 1B. In such a ring oscillator type VCO, the odd number-staged inverter circuits are cascade-connected to each other, thereby constituting the positive feedback loop, and the control voltage Vc is applied to the inverter circuits at the respective stages. It should be noted that a capacitor indicates a load capacitance. The Japanese Laid-open patent application No.2-53304 laid open in 1990 discloses such a conventional ring oscillator type VCO constructed of bipolar IC. To the contrary, another conventional ring oscillator type VCO constructed of MOSIC uses inverter blocks as generally illustrated in Fig. 1B, and arranged such that each stage of the inverter circuit is constructed of a single inverter made of the load transistor Q L and the driver transistor Q D , and then the control voltage Vc is applied to the load transistor Q L . An oscillator of that type is known from IBM Technical Disclosure Bulletin, vol. 31, no. 2, July 1988, pages 154 to 156. Fig. 2 is a waveform chart for representing an input signal "Vin" and an output signal "Vout" of one inverter. In the above-explained conventional ring oscillator type VCO using inverter blocks as shown in Figs. 1A and 1B, when the input signal Vin is fallen from the "high" level into the "low" level, the charging operation to the load capacitance of the next stage is commenced by the load transistor Q L . In connection with this charging operation, the level of the output voltage Vout is increased with the inclination (namely, increasing speed) corresponding to the control voltage Vc, and gradually reaches the voltage level of the power source. Then, this source voltage level of the output signal Vout is maintained for a moment. Thereafter, this input signal Vin is raised from the "low" level to the "high" level. Accordingly, the source voltage level (namely high level) of the output signal Vout is inverted into the low level. It should be understood that although a certain delay is actually produced between the rising (from LOW to HIGH) operation of this input signal Vin and the falling operation (from HIGH to LOW) of the output signal Vout, this delay amount is assumed to be zero for the sake of convenience. Then, while the level of the input signal Vin is high, the level of the output signal Vout is maintained at 0V. When the input signal Vin is fallen from the high level to the low level, the load capacitance is charged by the load transistor Q L , so that the output signal Vout is increased at a certain increasing speed. Then, the increasing (rising) speed of the output signal Vout from the level of 0V to the voltage level of the power source can be varied by controlling the control voltage Vc. As a consequence, the phase and the oscillating frequency can be changed by leading and delaying the timings in response to the control voltage Vc, at which the output voltage Vout from the inverter circuit rises from OV and intersects the threshold voltage of the subsequent-staged inverter circuit. On the other hand, in accordance with the conventional ring oscillator type VCO using inverter blocks as shown in Fig. 1B, only the delay amount of the rising edge of the output signal waveform within a single stage of inverter circuit can be controlled based on the control voltage Vc, but the delay amount of the falling edge thereof cannot be controlled. Then, in the conventional ring oscillator type VCO using inverter blocks as represented in Fig. 1B, the p-channel MOS transistor is controlled as the load transistor (variable current source) by the control voltage Vc. Alternatively, the n-channel MOS transistor may be controlled as this load transistor based on the control voltage Vc. In this alternative example, the delay amount of the falling edge of the output signal waveform may be controlled. However, conversely, the delay amount of the rising edge cannot be controlled. In any conventional inverter circuits, only a delay amount of either the rising edge, or the falling edge of the output signal waveform can be controlled. As a consequence, the edge delay control for the output signal waveform can be done only for every second stage, so that the duty ratio of the oscillated output signal waveform would be varied by the control voltage Vc. Thus, it is very inconvenient in such a case that this conventional ring oscillator type VCO is utilized as the digital type phase comparator of the PLL circuit, and the delay amounts of both the rising edge and the falling edge of each stage of the inverter circuit would be directly reflected into the resolution of this phase comparator. As a result, no high-precision PLL control operation can be achieved. There is another problem that the conventional ring oscillator type VCO cannot be utilized in such usage which requires the secondary distortion (duty ratio) characteristic of the oscillator output. Object and Summary of the Invention The present invention has been made in an attempt to solve these problems, and therefore, has an object to provide a ring oscillator circuit comprising a plurality of inverting circuits cascade connected to each other, each inverting circuit comprising: an even number of first inverters, each having a first transistor and a second transistor series-connected to said first transistor, in which an input signal is applied to an input terminal of one of said first and second transistors, a control signal is supplied to an input terminal of the other of said first and second transistors, and outputs of said first and second transistors are commonly connected to each other to form the output of the first inverter, and a second inverter having a third transistor and a fourth transistor series-connected to said third transistor, in which input terminals of said third and fourth transistors to which an output signal from one of said first inverters is inputted are commonly connected to each other, and output terminals said third and fourth transistors are commonly connected to each other to form the output of the second inverter. The ring oscillator type VCO (voltage-controlled oscillator) comprising such a ring oscillator circuit has said plural stages of inverting circuits being cascade-connected to each other, thereby constituting a positive feedback loop; wherein said plural stages of inverting circuits are arranged as three stages of inverters constructed of load transistors and driver transistors; and said control voltage is applied to the load transistors of two adjoining inverters among said three stages of inverters. In so doing delay amounts of both a rising edge of an output signal derived from an inverter circuit and a falling edge of this output signal can be controlled with having similar amounts, and the delay amounts of these rising edge and falling edge can be controlled so as not to vary the duty ratio of an oscillator output signal. Accordingly, in accordance with the ring oscillator type VCO of the present invention, since a single inverter circuit is arranged by the three-staged inverters and the load transistors of the two adjoining inverters among the three-staged inverters are controlled by the control voltage, the delay amounts of the three-staged inverters can be controlled as follows: The same delay amount can be achieved in such a manner that one of the rising edge of the output signal waveform and the falling edge thereof is controlled by way of one of these two adjoining inverters, and the other of these rising edge and falling edge is controlled by way of the other of these two adjoining inverters. Therefore, the delay amounts of the rising edge as well as the falling edge of the output signal waveform can be controlled without producing any variations in the duty ratio. Brief Description of The Drawings The above-described aspects and other features of the present invention will now be explained in the following description, taken in conjunction with the accompanying drawings in which: Figs. 1A and 1B are circuit diagrams for showing the inverter block employed in the conventional ring oscillator; Fig. 2 is a waveform chart for representing operations of the related art ring oscillator shown in Fig. 1; Fig. 3 is a schematic circuit diagram of an inverter block according to a first embodiment of the present invention; Fig. 4 shows one example of a circuit arrangement embodying the inverter according to Fig.3; Fig. 5 is a waveform chart for explaining operations of the inverter circuit shown in Fig. 4; and Fig. 6 is a schematic block diagram for showing a ring oscillator type VCO according to the present invention which uses inverter blocks according to Figs.3 and 4. Detailed Description of the Preferred Embodiments A ring oscillator type VCO (voltage-controlled oscillator) according to the present invention will now be described in detail with reference to various embodiments shown in the drawings. Figs. 3, 4 and 5 represent a single (three-stage) inverter circuit 10 and signal waveforms of this inverter of a ring oscillator type VCO, which constitutes a first embodiment. That is, Fig. 3 represents a schematic circuit arrangement of this three-staged inverter circuit, whereas Fig.4 indicates a concrete circuit diagram of this inverter circuit. Fig. 5 shows a waveform chart of this inverter circuit. A ring oscillator type VCO may be so constructed that plural stages of the inverter circuit 10 are employed to constitute a positive feedback loop, and then control voltages "Vc" applied to the respective stages of this inverter circuit are controlled. As to this circuit arrangement, the basic arrangement thereof is common to that of the conventional inverter circuit shown in Figs. 1A and 1B. However, the inverter circuit 10 of the ring oscillator type VCO, according to the first embodiment, has such a feature that, as shown in Fig. 3, the three stages of inverters are cascade-connected. This featured circuit arrangement of the first embodiment is completely different from the inverter circuit of the conventional ring oscillator type VCO. Each of these inverters which constitute the inverter circuit 10 is arranged by a series-connected circuit of a p-channel MOS transistor and an n-channel MOS transistor, which are connected between a power source voltage terminal and the ground. A capacitor shown in Fig. 3 indicates a load capacitance. Then, the p-channel MOS transistors of the first-staged inverter and the second-staged inverter are commonly controlled as load transistors Q L and Q L by way of a control voltage Vc. The n-channel MOS transistors of the first-staged and second-staged inverters commonly function as driver transistors Q D and Q D . Then, the third-staged inverter is arranged by the normal CMOS transistor in which no delay control is carried out. In such an inverter circuit, the rising speed is changed when a signal "Va" at a node "a" is increased, or raised from OV to Vcc levels, depending upon the magnitude of the current flowing through the load transistor of the first-staged inverter. The larger this current flows, the higher the rising speed becomes. As a consequence, the timing of the rising edge of the output signal Vout would become earlier. Also, the rising speed is changed when a signal "Vb" of the node "b" is increased from OV to Vcc levels, depending upon the magnitude of the current flowing through the load transistor QL of the second-staged inverter. The larger this current flows, the higher the rising speed becomes. As a result, the timing of the falling edge of the output signal Vout would become earlier. Accordingly, in accordance with the first embodiment, both of the rising edge of the output signal Vout and the falling edge thereof are delayed in a similar manner, so that the phase of this output signal Vout can be controlled. Then, since both of the rising edge of the output signal Vout and the falling edge thereof may be delayed by the same delay amount, the duty ratio of the output signal Vout can be continuously maintained at a constant value irrelevant to variations in the control voltage Vc, for instance, 50%. As a consequence, the inverter circuit 10 of this first embodiment could be employed without any problem in such a case that the delay amounts of both the rising edge of the output signal Vout and the falling edge thereof may directly cause the resultant resolution of the phase comparator, as in the conventional inverter circuit. Furthermore, the secondary distortion (duty ratio) characteristic of the oscillator output can be utilized in the required usage. It should be noted that although the p-channel MOS transistors of the first-staged inverter and the second-staged inverter employed within the inverter circuit 10 of the above-explained first embodiment are used as the load transistors (variable current sources) and the n-channel MOS transistors are employed as the driver transistors, the concept of the present invention may be realized by employing such a circuit arrangement that the n-channel MOS transistors of the first-staged and second-staged inverters are employed as the load transistors (variable current sources), and the p-channel MOS transistors thereof are utilized as the driver transistors. Furthermore, although the delay amounts of the rising edge and the falling edge are controlled by way of the first-staged and second-staged inverters within the inverter circuit 10 in accordance with the above-explained first embodiment, the delay amounts of the rising edge and also the falling edge may be controlled by way of the second-staged and third-staged inverters. As described above, the present invention may be embodied by way of various modes. Fig. 6 is a schematic circuit diagram of another embodiment of a ring oscillator type VCO according to the present invention. In this ring oscillator type VCO, the inverters equipped with the variable current sources controlled by the control voltage Vc (namely, controlled by the transistors Q L ) shown in Fig. 3 and Fig. 4 are utilized as a single inverter unit 10, such inverter units 10 are cascade-connected to each other, and load capacitances are added to the respective connection terminals of the cascade-connected inverter units. Furthermore, a portion of the output signal derived from the final-staged inverter unit is positive-fed back to the input of the first-staged inverter unit. These final-staged inverter unit and first-inverter unit are cascade-connected to each other. In this circuit arrangement of Fig. 6, the control voltages Vc are applied to the respective inverter units 10 so as to vary the currents flowing through the inverters of the respective inverter units 10, and also to vary time required to charge/discharge the capacitors, so that the delay times may be controlled. As a result, the frequency of the oscillator can be varied. In other words, this circuit may be operated as the VCO (voltage-controlled oscillator) circuit with using the ring oscillator. The ring oscillator type VCO according to the present invention has such a feature that the inverter circuit 10 is arranged by the three-staged inverters constructed of the load transistors and the driver transistors, and the control voltages are applied to the load transistors of the two adjoining inverters within the three-staged inverters. As a consequence, in accordance with the ring oscillator type VCO of the present invention, since a single inverter circuit 10 is arranged by the three-staged inverters, and also the load transistors of the two adjoining inverters within the three-staged inverters are controlled by the control voltages, the delay amounts of the three-staged inverters can be controlled as follows: The same delay amount can be achieved in such a manner that one of the rising edge of the output signal waveform and the falling edge thereof is controlled by way of one of these two adjoining inverters, and the other of these rising edge and falling edge is controlled by way of the other of these two adjoining inverters. Therefore, the delay amounts of the rising edge as well as the falling edge of the output signal waveform can be controlled without producing any variations in the duty ratio.

Claims: A ring oscillator circuit comprising a plurality of inverting circuits (10) cascade-connected to each other, each inverting circuit comprising: an even number ot first inverters, each having a first transistor (Q L ) and a second transistor (Q D ) series-connected to said first transistor (Q L ), in which an input signal (Vin) is applied to an input terminal of one of said first and second transistors, a control signal (Vc) is supplied to an input terminal of the other of said first and second transistors, and outputs of said first and second transistors are commonly connected to each other for forming the output of said first inverter, and a second inverter having a third transistor (Q P ) and a fourth transistor (Q n ) series-connected to said third transistor (Q P ), in which input terminals of said third and fourth transistors to which an output signal from one of said first inverters is inputted are commonly connected to each other, and output terminals of said third and fourth transistors are commonly connected to each other to form the output of said second inverter. A ring oscillator type VCO, or voltage-controlled oscillator, comprising a ring oscillator circuit in accordance with claim 1, in which said plural stages of inverting circuits are cascade-connected to each other, thereby constituting a positive feedback loop; wherein said plural stages of inverting circuits are arranged as three stages of inverters constructed of load transistors (Q L ) and driver transistors (Q D ) ; and said control voltage (Vc) is applied to the load transistors of two adjoining inverters among said three stages of inverters. A ring oscillator type VCO as claimed in claim 2, wherein: each of said adjoining inverters comprises a p-channel MOS transistor and an n-channel MOS transistor; and said p-channel MOS transistor is the load transistor (Q L ) having a gate to which the control voltage (Vc) is applied. A ring oscillator type VCO as claimed in claim 2, wherein: each of said adjoining inverters comprises a p-channel MOS transistor and an n-channel MOS transistor; and said n-channel MOS transistor is the load transistor (Q L ) having a gate to which the control voltage (Vc) is applied.

Inventors: GOSEKI MASAMI SONY CORPORATION, OGAWA AKIHIDE SONY NAGASAKI CO, TAKEDA KAZUHIRO SONY NAGASAKI, GOSEKI, MASAMI, SONY CORPORATION, OGAWA, AKIHIDE, SONY NAGASAKI CORPORATION, TAKEDA, KAZUHIRO, SONY NAGASAKI CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: H03K3
IPC codes: H03K3, H03L7